i8o 



COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



series of organs in the adult. At points on these lines the sensory 

 areas are developed by the differentiation of two kind of cells, the 

 supporting cells which extend through the epidermis from the corium 

 to the free surface, and the sensory cells which reach from the surface 

 only part way to the base. The latter are pear-shaped and bear 

 cuticular hairs or bristles on their free ends (fig. 179), while the deeper 

 ends are embraced by the non-medullated fibrils of the lateralis system 

 of nerves, which follow the lines of organs, and in development keep 

 pace with their extension. These sensory areas are the nerve hillocks 

 or neuromasts alreadv referred to. 



FIG. 179. FIG. 180. 



FIG. 179. Sense organ of lateral line of Diemyctylus (aquatic form) freely after Kings- 

 bury, C, cone cells; 5, spindle cells. 



FIG. 180. Developing lateral line organ on one side of head of Amia, showing method 

 of closure of grooves to canals, after Allis. an, anterior naris; io, so, infra- and supraorbital 

 lines; pn, posterior naris. 



In the cyclostomes and aquatic amphibia each sensory patch 

 sinks into a separate pit (fig. 179), but in all other itchhyopsida the 

 lines of organs sink in the same way, the patches being connected by 

 grooves. In Chim&ra these grooves remain open, but in all others they 

 are closed except at certain points where pores connect the canals 

 formed by the closed grooves with the exterior. In this way the sensory 

 areas come to lie in canals beneath the surface, water obtaining access 

 to them through the pores. In many teleosts (fig. 181) the pores 

 pass through notches or openings in the scales, while on the head the 

 canals themselves frequently run through some of the cranial bones. 



Of considerable morphological importance, especially in connection with the 

 morphology of the ear, are the facts that the sensory areas multiply by elongation, 

 followed by division, and that the pores themselves increase in the same way 



